1. Technical Field
This invention relates to PET containers having enhanced thermal properties and methods for making the same.
2. Background Art
Blow molding processes for forming PET containers are well known in the art. Blown PET containers have replaced metal and glass containers in numerous food storage applications such as carbonated soft drinks and lower temperature filled food products such as peanut butter and mayonnaise. However, prior art PET containers have not replaced metal and glass containers for product storage and processing applications where the container is filled or heated to temperatures above 97° C. (207° F.) as such containers experience significant shrinkage, deformation rendering the container unuseable. Additional in-roads into the replacement of glass is desired in food processing applications such as low-temperature pasteurization, high-temperature pasteurization and retort. Low temperature pasteurization include the pasteurization of liquid products such as beer and tea. High temperature pasteurization processes are for solid food products such as pickles that have slower heat transfer and require temperatures in excess of 100° C. Retort processes are for pasteurizing low acid products and require temperatures from 100° C. to 130° C. and pressures sufficient to maintain water in a liquid state.
Prior art efforts to increase the thermal performance of PET containers have focused on increasing the crystallinity levels of PET. PET is a crystallizable polymer meaning that its crystallinity can be manipulated by the process of forming articles from the PET. These efforts have been successful to the extent of forming PET containers capable of withstanding temperatures up to 97° C. (207° F.) but not much beyond.
A two-phase model of PET states that PET molecules can exist in two morphologies: an amorphous phase and a crystalline phase. The amorphous phase has been described on a molecular level as resembling a bowl of spaghetti. In a solid state the molecular motion is restricted to very short range vibrations and rotations but in the molten state there is considerable segmental motion arising from rotation about chemical bonds.
In the crystalline phase the polymer chains arrange themselves in thermodynamically favorable alignment. Crystalline portions of the PET molecules can extend straight in one direction and then fold back and forth numerous times to form a folded structure. Numerous such folded structures can stack to form more complex structures known as lammelae. Parallel chains in the crystalline phase can be connected with reentry folds of amorphous portions of the molecule in what is known as a switchback model.
A three phase model of PET has also been proposed to account for deficiencies observed in the two-phase model. The three-phase model includes a crystalline phase, a rigid amorphous phase and a mobile amorphous phase. One article describing the three-phase model is “Vitrification and devitrification of the rigid amorphous fraction in polyethylene terephthalate)” by Maria Cristina Righetti and Maria Laura Di Lorenzo published at e-polymers.org in 2009, the disclosure of which is incorporated herein in its entirety and made a part hereof.
Three commonly known methods for increasing the crystalline fraction of PET includes quiescent crystallization, strained induced crystallization and a combinations of the two. Quiescent crystallization requires exposing an amorphous PET article to heat above the glass transition temperature of PET (70° C. or 158° F.) to impart mobility into the polymer chains to allow them to reorganize into the crystalline morphology. Strain induced crystallization requires stretching of the PET under proper heat and extension ratios to orient the PET molecules into a crystalline morphology. An example of strain induced crystallization is when a preform (a test tube shaped article) is blown into a mold of greater volume to cause stretching of the preform in a single direction or in multiple directions to cause strain-induced crystallization in the expanded article. Articles with strain induced crystallinity can be exposed to heat in a process known as heat setting or thermal annealing to cause a relaxation in the stressed induced crystallinity to increase the thermal properties of the final article. The prior art discloses that the orientation of the polymer chains creates a condition where crystal formation is kinetically favorable upon application of thermal energy.
PET blow mold systems can be an integrated system or a non-integrated system. An integrated system includes an injection molding station for forming the preform in-line with the blow mold station. The preform coming from the injection mold does not have to be reheated and may have to be cooled to the desired orientation system. In a non-integrated system, the preform in injection molded, cooled and then fed into the blow mold station or stations where it is reheated to the desired orientation temperature and then conveyed to the blow mold station or stations.
U.S. Pat. Nos. 4,476,170; 4,512,948; 4,522,779; 4,535,025; 4,603,066; 4,713,270; 4,839,127 and 4,891,178 disclose single mold systems for forming PET containers. As these patents name Jabarin as an inventor they shall sometimes be referred to as the Jabarin patents. Those patents disclose using mold temperatures up to 250° C. (482° F.) to form containers having crystallinities of up to 60%. Removing the finished containers from such molds without shrinkage of the containers requires either lowering the temperature of the mold to a point where the containers are self-sustaining and can be removed or applying internal pressure to the container when removing the container until the container cools to a temperature where the container is self-sustaining. As explained by Dr. Timothy J. Boyd in his dissertation “Transient Crystallisation of Poly (Ethylene Terephthalate) Bottles” (“Boyd Dissertaion”) neither of these techniques were commercially feasible as the first technique would require extremely long cycle times and the second would be difficult to control in commercial applications.
U.S. Pat. Nos. 5,562,960 and 5,501,590 disclose two mold systems for forming PET containers known as a dual-blow system. Those patents require forming an intermediate article in a first mold having a volume greater than the finished container, conveying the intermediate article through a shrink oven to crystallize the intermediate article and then placing the intermediate article into a second mold where it is blown into the finished article. Containers formed from this method have reported crystallinities from 40-50%.
U.S. Pat. Nos. 6,485,669; 6,485,670; 6,514,451; 6,749,415 and 6,767,197 (“Boyd et al. patents”) and the Boyd Dissertation disclose that the minimum amount of cooling during the blow molding process and the higher the temperature at de-molding leads to the higher thermal properties of the finished article. The Boyd et al. patents disclose blowing heated air, hot air annealing, or a combination of heated air and fluid onto the inner surface of an article in a blow mold to increase the thermal properties of the finished article.
Commercial techniques for forming PET utilize both threaded and unthreaded preforms. Preforms are essentially amorphous having less than about 5% crystallinity. Upon blow molding a threaded preform into an expanded article the threads will have substantially the same dimension in the finished article as the preform, and, therefore, will have little if any strain induced crystallization. Such a finish will be susceptible to softening and deformation under hot fill conditions. Thus, some amount of crystallization must be imparted to the finish section to enhance thermal performance without shrinking the finish and without imparting whitening to the finish. U.S. Pat. No. 7,033,656 discloses a method for crystallizing the finish section in such a way that one surface is crystallized throughout its length and the other surface includes an area that is essentially uncrystallized with a crystallization in a mid-portion of the finish being graded between the surfaces.
U.S. Pat. No. 4,233,022 discloses an apparatus for forming a PET container from a threaded preform. The '022 patent states that due to the low orientation of the finish and the heel of the container during blow molding that it is undesirable to heat set these areas as it would create whitening in these areas by creating sperulitic crystallinity. Thus, the '022 patent discloses a blow station that selectively heats the strain-oriented sections of the container and cooling the portions of the container having little or no strain orientation.
U.S. Pat. No. 6,841,117 discloses a method for blow molding a container from an unthreaded preform. The method includes the step of blow molding a preheated, threadless preform in a heated mold having threads of the desired size to form an intermediate container having threads. The intermediate container has a moil section above the threaded finish which is cut from the intermediate container to form the final container. The finish will have a desired crystallinity of 25% to provide sufficient thermal properties for hot fill applications. More particularly, the preform is preheated to a temperature of 108° C. and then disposed within a mold cavity maintained at temperatures from 138-143° C. The portion of the mold cavity forming the bottom of the container is maintained at 49-54° C. After the mold is closed the preform is blown with air pressure of 40 bar for 1.5 to 3 seconds.
A stretch cooling rod blows recirculating cooling gas at a temperature from 20-40° C. inside the container in the region of the blown threads. The container is removed from the mold at below about 80° C.
These and other aspects and attributes of the present invention will be discussed with reference to the following drawings and accompanying specification.